In a non-linear operational transconductance amplifier the output current is a non-linear function of the input signal level, increasing at a faster rate than the signal at higher signal levels. Typically, such non-linear effect is readily achieved by the introduction of a resistance in a current mirror amplifier (CMA) which forms part of the signal gain path.
This is best explained with reference to the prior art circuit of FIG. 1 which shows a known operational transconductance amplifier. Referring to FIG. 1, the circuit includes a long-tailed differential pair of transistors (T1, T2) whose output currents (I1, I2) form the respective input currents of first (CMA1) and second (CMA2) current mirror amplifiers. The first current mirror amplifier (CMA1) produces a first output current (IO1), of first polarity relative to an output terminal (OUT1), which is applied to the output terminal (OUT1) of the operational transconductance amplifier. The second current mirror amplifier (CMA2) produces an output current IO2A which is mirrored by a third current mirror amplifier (CMA3) to produce an output current IO2 which is applied to the output terminal, and which is of opposite polarity to the first output current relative to the output terminal. That is, for the circuit of FIG 1, N1 "sinks" a current IO1 out of OUT1 and N3 "sources" a current IO2 into OUT1. Under conditions of zero differential signal input to the differential pair of transistors, the quiescent currents (IO1 and IO2) of the first and third current mirror amplifiers are equal and satisfy one another and thereby cause the output terminal to supply zero current to a load connected to OUT1. When a differential input signal causes the output currents of the differential pair of transistors to be unequal, the output currents IO1 and IO2 of are no longer equal and a net signal-current flows into or out of a load (not shown) connected to the output terminal (OUT1).
In a non-linear operational transconductance amplifier, typically each one of the first and second current mirror amplifiers includes an input diode-connected transistor (e.g., NA, NB) whose current is mirrored via an output transistor (e.g., N1, N2). Each one of the first and second CMAs are normally arranged to be non-linear by the introduction of an emitter series resistance (e.g., RA, RB) in the input diode-connected transistor or "master diode" side of the current mirror amplifier. Usually, the input diode-connected transistor (e.g., NA, NB) of the CMA has a larger size geometry than the output transistor (e.g., N1, N2), and the zero differential signal quiescent current in its emitter resistor produces a voltage drop which assures that the amplifier operates in a non-linear mode at a predetermined current level. Typical circuit applications are often near-optimal with similar current levels in the differential and output stages at zero signal. When the voltage drop which the input current causes across an emitter series resistance increases at higher signal levels, the output-current to input-current ratio (that is, the current gain) is caused to increase, providing an output drive capability and efficiency substantially exceeding that of a conventional linear amplifier.
Such an arrangement functions so long as the transistor current gains are very high, that is, so long as the base current requirements are negligibly small compared with the operating current levels.
In practice, typical transistor current gains may cause problems to arise. This can be understood from an example. For instance, consider the condition where the non-linear operational transconductance amplifier of FIG. 1 utilizes current mirror amplifiers wherein the input-to-output transistor geometry ratio of NA to N1 and NB to N2 is 10:1 (e.g., NA is ten times the size of N1). Assume further that under balanced conditions (i.e., I1=I2) and at a prescribed tail current IT of 2.sub.o (where I.sub.o =I.sub.1 =I.sub.2) a voltage drop is produces across RA and RB which is equal to (KT/q)(ln10)=60 mV at 25.degree. C., where K is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electrical charge on an electron. Under balanced conditions, the effect of the geometry ratio of 10:1 to reduce the current gain is counterbalanced by the prescribed voltage drop of 60 mV across the emitter series resistance, so that the resulting current gain will be unity.
Now, consider the situation with an input signal to the nonlinear operational transconductance amplifier sufficiently large so that all of the tail current flows through one transistor (e.g., T1). The input current to the corresponding current mirror amplifier (e.g., CMA1) is then doubled, from I.sub.o to 2I.sub.o. The voltage drop across the emitter series resistance (e.g., RA) will also double to 120 mV. Furthermore, the forward drop across the diode-connected input transistor (e.g., NA) of the current mirror amplifier (e.g., CMA1) will increase by 18 mV due to the doubling of the current through it. In accordance with well-known principles, the current gain of the current mirror amplifier will be 10 and its output current can increase to 20 times its zero differential signal current. Under these conditions, the base current requirement of the output transistor will be 20 times its quiescent base current requirement. In the current mirror amplifier configuration shown in FIG. 1, the base current requirement of the output transistor is directly supplied by the input current to the current mirror amplifier. Under quiescent conditions the base current requirement may well be a negligible demand upon the input current. However, 20 times that demand will, in general, interfere with proper operation to an undesirable extent. Thus, when such a current is subtracted from the input current, the postulated voltage drop of 120 mV across the emitter series resistance will be significantly reduced and, accordingly, the desired increase in current gain will not be realized for the current mirror. While it is possible to increase the size of the output device in order to reduce current gain fall-off at higher currents, the input device must be further proportionately increased in size in order to maintain the desired geometry ratio and it thereby tends to become unreasonably large.